1. Field of the Invention
This invention is directed to solid electrolytes containing lithium bis(trifluoromethane sulfonyl)imide, a solvent and, in particular, a solvent comprising a mixture of triglyme and an organic carbonate. This invention is further directed to solid electrolytic cells (batteries) containing an anode, a cathode and a solid electrolyte containing lithium bis(trifluoromethane sulfonyl)imide, a solvent comprising a mixture of triglyme and an organic carbonate. This invention is also directed to methods for enhancing the cumulative capacity of the solid electrolytic cells by employing a solid electrolyte which contains lithium bis(trifluoromethane sulfonyl)imide, a solvent comprising a mixture of a organic carbonate and triglyme.
2. State of the Art
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety features. Notwithstanding their advantages, the use of these solid batteries over repeated charge/discharge cycles is substantially impaired because these batteries typically exhibit significant drops in their charge and discharge capacity over repeated cycles as compared to their initial charge and discharge capacity.
Specifically, solid batteries employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix as well as a suitable inorganic ion salt. The inorganic matrix may be non-polymeric [e.g, .beta.-alumina, silver oxide, lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazine) polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic monomers include, by way of example, polyethylene oxide, polypropylene oxide, polyethyleneimine, polyepichlorohydrin, polyethylene succinate, and an acryloyl-derivatized polyalkylene oxide containing an acryloyl group of the formula CH.sub.2 .dbd.CR'C(O)O-- where R' is hydrogen or lower alkyl of from 1-6 carbon atoms.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix. Nevertheless, electrolytic cells containing a solid electrolyte containing a polymeric matrix suffer from low ion conductivity and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed into a very thin film, i.e., on the order of about 25 to about 250 .mu.m. As is apparent, the reduced thickness of the film reduces the total amount of internal resistance within the electrolyte thereby minimizing losses in conductivity due to internal resistance.
The solid electrolytes also contain an inorganic ion salt and a solvent (plasticizer) which is typically added to the matrix in order to enhance the solubility of the inorganic ion salt in the solid electrolyte and thereby increase the conductivity of the electrolytic cell. In this regard, the solvent requirements of the solvent used in the solid electrolyte are art recognized to be different from the solvent requirements in liquid electrolytes. For example, solid electrolytes require a lower solvent volatility as compared to the solvent volatilities permitted in liquid electrolytes.
Suitable solvents well known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like.
A. Webber, J. Electrochem. Soc., 138 (1991) 2586, reports the use of lithium bis(trifluoromethane sulfonyl)imide in the liquid electrolyte systems propylene carbonate/1,2-dimethoxymethane, and propylene carbonate/1,2-dimethoxymethane/1,3-dioxolane. J. T. Dudley et al., J. Power Sources, 35 (1991) 59, investigated the conductivities of several salts including lithium bis(trifluoromethane sulfonyl)imide in various solvents. L. A. Dominey et al., Proc. Intersoc. Energy Convers. Eng. Conf. 25 (1990) 382, report a liquid electrolyte containing lithium bis(trifluoromethane sulfonyl)imide in tetrahydrofuran, or the polymer poly(methoxyethoxy methoxyethoxy-ethoxyphosphazene). U.S. Pat. No. 5,021,308 discloses cells using liquid electrolytes composed of lithium bis(trifluoromethane sulfonyl)imide in an aprotic liquid solvent mixture of propylene carbonate and ethylene carbonate or dimethoxy methane. U.S. Pat. Nos. 4,851,307 and 5,063,124 disclose the use of lithium bis(trifluoromethane sulfonyl)imide as a salt in a sulfonated solvent, which combination finds use as an ionically conductive material in an electrochemical cell. The ionically conductive material may contain a macromolecular material. U.S. Pat. No. 5,162,177 discloses liquid electrolytes consisting of an aprotic solvent, for example propylene carbonate and ethylene carbonate, and a lithium bis(trifluoromethane sulfonyl)imide salt. A mixed solvent of propylene carbonate and dimethoxy methane was also used. U.S. Pat. No. 5,072,040 discloses several methods for the synthesis of lithium bis(trifluoromethane sulfonyl)imide. U.S. Pat. No. 4,505,997 discloses a method of making lithium bis(trifluoromethane sulfonyl)imide, in solid solutions with poly(propylene oxide) and poly(ethylene oxide) which have cationic conductivity enabling their use as electrolytes in rechargeable cells. Fiona M. Gray "Solid Polymer Electrolytes", VCH Publishers, Inc. New York, N.Y. (1991) pages 7, 107 and 117, discloses lithium bis(trifluoromethane sulfonyl)imide is a salt with possible applications in polymer electrolytes. Gray distinguishes polymer electrolytes per se from systems containing low-molecular weight solvents as well as polymers. Gray teaches that in the former case net ionic motion takes place without long-range displacement of the solvent, ion transport relies on local relaxation processes in the polymer chain which may provide liquid-like properties. Gray also reports that CH.sub.3 (OCH.sub.2 CH.sub.2).sub.n OCH.sub.3 has been added to poly(ethylene oxide)-based electrolytes to improve their conductivity, but the mechanical properties of the materials were poor. The disclosures of each of the foregoing references is incorporated herein in its entirety.
The solid, solvent-containing electrolyte is typically formed in one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic ion salt and the electrolyte solvent (i.e., the triglyme of Formula I and the organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte.
In the other method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery (i.e., an electrolytic cell).
Notwithstanding the above, the initial capacity of solid batteries is often less than desirable. Moreover, even when the initial capacity of the solid battery is relatively high, such solid batteries often exhibit rapid decline in capacity over their cycle life.
Specifically, the cumulative capacity of a solid battery is the summation of the capacity of a solid battery over each cycle (charge and discharge) in a specified cycle life. Solid batteries having a high initial capacity but which rapidly lose capacity over the cycle life will have low cumulative capacity which interferes with the effectiveness of these batteries for repeated use.
In view of the above, the art is searching for methods to enhance the cumulative capacity of such solid batteries. It goes without saying that increases in the cumulative capacity of solid batteries would greatly facilitate their widespread commercial use.
Another particular problem in the development of solid polymer electrolytes is the poor ambient temperature (0.degree.-60.degree. C.) conductivity of the electrolyte.
Still another problem of solid polymer electrolytes is the presence of dangerous or thermally unstable inorganic ion salts. It would be advantageous if the solid electrolyte were highly conductive at ambient temperatures, safe and thermally stable, as well as, capable of enhancing high cumulative capacity in solid batteries.